Grow Room Glossary

VPD

The difference between how much water vapour the air is carrying and how much it could carry at that temperature. High VPD = dry air, plants transpire fast. Low VPD = humid air, plants slow down.

Targets: Seedling 0.4–0.8 kPa, Veg 0.8–1.2 kPa, Flower 1.0–1.5 kPa.

BudMaster calculates VPD from temperature + humidity in real time and uses it to drive fans, heaters and dehumidifiers. See our free VPD calculator.

RH

How full the air is with water vapour, expressed as a percentage of saturation at the current temperature. 100% RH means the air can hold no more water. RH alone is a poor measurement for growing — always pair it with temperature and look at VPD.

Dew Point

The temperature at which the air becomes saturated and water starts to condense. Critical for avoiding damp patches on walls, cool spots in the canopy, and condensation inside the controller enclosure. If your dew point is close to your coolest surface temperature, you have a mould risk.

Leaf Temperature

The temperature of the plant’s leaves, measured with an infrared thermopile. Usually 1–3°C below air temperature because transpiring leaves cool themselves. In CO₂-enriched sealed rooms, leaves can be 3–5°C cooler than air — and that changes the effective VPD at the leaf surface.

CO₂

Atmospheric CO₂ is around 420 ppm. Many growers enrich to 800–1500 ppm during lights-on to boost photosynthesis and yield. Above ~5000 ppm it becomes a health hazard. CO₂ enrichment only pays off if your light and VPD are already dialled in.

Measured by NDIR sensors (see below). The BudMaster Nexus hub includes dual-safety CO₂ dosing — software timeout plus a separate hardware cutoff.

Heat Index

Combines temperature and humidity into a single "perceived temperature" value — the same formula used in weather forecasts. Useful for understanding how plant-stressful the room feels vs the thermometer reading alone.

PAR

The band of visible light your plants actually use for photosynthesis. Everything outside this range is either wasted as heat or useful only for signalling. Not a measurement itself — just the range that PPFD and DLI are measured across.

PPFD

Instantaneous light intensity in the PAR range, measured at the canopy. Essentially "how many photons hit each square metre every second." The number that shows on a PAR meter.

Rough targets at canopy: Seedling 100–300, Veg 400–600, Flower 700–1000, CO₂-enriched flower 1200–1500.

DLI

Total PAR photons delivered to your canopy across a whole day. PPFD integrated over the photoperiod.

DLI = PPFD × hours × 3600 ÷ 1,000,000

Targets: Seedling 12–18, Veg 25–40, Flower 40–50 mol/m²/day.

DLI is what actually drives yield — more mol/m²/day = more photosynthesis = more biomass. BudMaster tracks DLI in real time and auto-dims your lights to hit the target for the stage.

Lux

Visible-light intensity weighted by human eye sensitivity. Useful for "is the light on?" but not for plants — plants don’t care what looks bright to a human. Use PPFD for canopy measurements. Lux is fine as a cheap proxy if you’ve calibrated it against a known PAR meter.

Photoperiod

How long the lights are on each day. Common indoor cycles: 18/6 for vegetative growth (18h light, 6h dark), 12/12 to trigger flowering or fruiting in short-day plants. Day-neutral cultivars don’t follow photoperiod cues — they can run 20/4 or 18/6 throughout their lifecycle.

Sunrise / Sunset Ramps

Gradually ramping light intensity up at the start of the day and down at the end, rather than a hard on/off. Reduces plant stress, emulates outdoor conditions, and gives stomata time to adjust. BudMaster ramps over 15–60 minutes on dimmable drivers.

Soil Moisture

Percentage of the soil volume occupied by water. Measured capacitively (electrical permittivity of the soil). Dry = 5–15%, field capacity = 30–45%, saturated = >60%. Avoid swinging between extremes — plants dislike drought-then-flood cycles.

EC

Measure of dissolved salts (i.e. nutrients) in the soil water. Higher EC = more nutrients available. Typical ranges: seedling 0.4–1.0, veg 1.2–2.0, flower 1.8–2.8 mS/cm. EC climbing over time usually means salt buildup — flush.

pH

How acidic or alkaline the root zone is. 7 is neutral. Soil growers target 6.0–6.8. Coco and hydro target 5.5–6.3. Outside those ranges, specific nutrients lock out regardless of what’s present — calcium at low pH, iron at high pH.

NPK

The three primary macronutrients. Nitrogen (N) drives leafy green growth, phosphorus (P) drives root and bloom development, potassium (K) drives overall plant health and flower quality. Most feed regimes skew high-N in veg, high-P-K in flower.

Industrial 7-in-1 RS-485 probes read N, P, K directly from soil — along with moisture, temp, EC and pH. The BudMaster Nexus hub supports them out of the box.

NDIR

The only reliable technology for CO₂ measurement in growing environments. Shines an infrared beam through an air sample; CO₂ absorbs a specific wavelength, the detector measures the absorption. Cheap "electrochemical" CO₂ sensors drift wildly and aren’t suitable. All BudMaster CO₂ readings come from NDIR.

AHT20

Factory-calibrated I²C sensor reading temperature to ±0.3°C and humidity to ±2%RH. Commonly paired with a BMP280 for pressure on a combo breakout board.

BMP280

Reads atmospheric pressure (hPa) with high precision. Useful for altitude compensation in CO₂ readings and as a weather-change tell. Often in the same package as an AHT20 temperature/humidity chip.

SCD41

Sensirion’s photoacoustic NDIR CO₂ sensor. Reads 400–5000 ppm CO₂ at ±40 ppm accuracy, plus on-chip temperature and humidity. What sits in the BudMaster triple-sensor pod.

TSL2591

High-dynamic-range photometric sensor reading from dim starlight (188 µlux) up to bright sunlight (88,000 lux). Used in BudMaster for lux, PAR approximation and DLI integration.

RS-485

Differential serial protocol rated for long cable runs (up to 1200 m) and noisy environments. Used for industrial soil probes that need to sit in wet soil for months. The BudMaster Nexus hub has RS-485 for 7-in-1 probes.

Modbus RTU

The most common industrial sensor protocol. Request-response, register-mapped, fault-tolerant. Every professional soil probe speaks Modbus. BudMaster implements a full Modbus master to talk to them.

ESP32-S3

Dual-core 32-bit MCU at 240 MHz with 16 MB flash, built-in WiFi and a battery of hardware peripherals. Runs the BudMaster firmware. Arduino-IDE compatible.

Transpiration

The process where water enters the plant through the roots, climbs the xylem, and exits as vapour through the stomata on the leaves. Drives nutrient uptake — nutrients dissolved in the water hitch-hike up with it. VPD is the key driver: higher VPD = more transpiration, up to a point.

Stomata

Tiny valves on the underside of leaves that open to let CO₂ in and water out. Plants close them when water is scarce (high VPD, drought, or heat stress) — which also stops CO₂ coming in. A plant with closed stomata is not growing.

Photosynthesis

Plants turn light + CO₂ + water into sugar + oxygen. Photosynthesis rate scales with PPFD up to a saturation point that depends on CO₂ and nutrients. More light is only useful if VPD, CO₂ and nutrients are keeping up.

Botrytis

A fungus (Botrytis cinerea) that rots flowers from the inside out in humid conditions — growers usually call it bud rot or grey mould. Develops fast at RH >60% with dew-point conditions on the canopy. Why late-flower VPD targets trend drier. BudMaster’s night-cycle intelligence runs a "bud-rot guard" that keeps humidity in the safe zone through lights-off.

Closed-loop control

Using measured feedback to adjust output — e.g. reading current DLI and dimming lights to hit a target. Opposite of "open-loop" (setting fans to 50% and hoping). Every BudMaster loop is closed-loop.

PID

The classic control algorithm that figures out how hard to push based on how far off target you are (P), how long you’ve been off (I) and how fast you’re approaching (D). Used for fan control, CO₂ dosing and heater regulation in BudMaster.

Night cycle

Lights-off period. Plants respire (consume rather than produce O₂), temperature drops, humidity usually climbs. A night-cycle-aware controller modulates fans and dehumidifiers differently from day — the targets shift.

Demand dosing (CO₂)

Opening the CO₂ solenoid only when CO₂ drops below target — not on a timer. Also called "closed-loop CO₂." Required for CO₂ enrichment that actually holds target rather than overshooting and venting.

OTA updates

Controller downloads new firmware over WiFi/Ethernet and installs it itself. No USB cable, no laptop, no disassembly. BudMaster ships with OTA over the local network — you click "update" in the dashboard and it’s done in 30 seconds.